Collaborative Research: Carbonation of Serpentinite in the San Andreas Fault: How Fluid-rock Interactions Impact Aseismic Creep

众所周知，圣安德烈亚斯断层的某些部分显示出频繁的小震级地震、偶尔的中等震级地震和无地震蠕变。其他地段没有抗震蠕变，但罕见的致命破裂，如1857年和1906年的地震。帕克菲尔德以北的圣安德烈亚斯断层上的高地震蠕变速率是由丰富的热液蚀变海岸地幔岩石的低剪切强度最好地解释的。尽管水化地幔岩石的主要矿物蛇纹石的机械强度太高，无法实现观测到的高置换速率，但人们越来越认识到，滑石在削弱圣安德烈亚斯断层方面发挥了关键作用。然而，圣安德烈亚斯断层滑石形成的机制、条件和后果仍然存在争议。该项目要检验的假设是，乔拉姆山谷和圣胡安包蒂斯塔之间高达28毫米/年的蠕动速度是由富含二氧化碳的流体与蛇纹石反应形成机械强度较弱的滑石和菱镁矿(皂石)控制的。此外，假设皂石与富二氧化碳流体的长期相互作用导致形成机械强度更强、速度更弱的石英(或蛋白石)和菱镁矿，从而在名义上较弱的蛇纹岩中促进微地震。为了验证这两种假设，该项目将把岩石学结果与流体包裹体分析和摩擦实验结合起来，以确定：1)矿物置换反应，2)形成条件，3)乔拉姆和圣胡安包蒂斯塔之间逐渐发生二氧化碳蚀变的海岸山脉岩石的地质力学特征。最后，将分析Parkfield以北SAF沿线的三维随时间变化的地震活动数据，以检查其与矿物学分区的相关性。加州经历了致命的高冲击地震，造成数百人死亡，造成数十亿美元的损失；因此，迫切需要了解流体-岩石相互作用如何影响圣安德烈亚斯断层系统的力学性质。这个项目探索了一种新的想法，可以解释为什么断层的某些部分表现出无地震蠕变，而其他部分则以地震为特征。最终，对蛇纹岩碳化作用的研究将促进对美国人口最稠密地区之一的地震活动和地震形变周期的更深入了解。

It is well established that some segments of the San Andreas Fault reveal frequent earthquakes of small magnitude, occasional earthquakes of moderate magnitude, and aseismic creep. Other segments feature no aseismic creep, but rare fatal ruptures, such as the 1857 and 1906 earthquakes. The high rates of aseismic creep along the San Andreas Fault north of Parkfield are best explained by the low shear strength of abundant hydrothermally altered Coast Range mantle rocks. While serpentine, the dominant mineral of hydrated mantle rock, is mechanically too strong to allow the observed high displacement rates, there is a growing recognition that talc plays a key role in weakening the San Andreas Fault. Yet the mechanisms, conditions and consequences of talc formation in the San Andreas Fault remain controversial. The hypothesis to be tested in this project is that the high creeping rates of up to 28 mm/yr between Cholame Valley and San Juan Bautista are controlled by the reaction of carbon dioxide-rich fluids with serpentine to form mechanically weak talc and magnesite (soapstone). In addition, it is hypothesized that prolonged interaction of soapstone with carbon dioxide-rich fluids causes the formation of mechanically much stronger and velocity-weakening quartz (or opal) and magnesite, promoting micro-earthquakes in a nominally weak serpentinite. To test both hypotheses this project will merge petrographic results with fluid-inclusion analysis and friction experiments to determine: 1) mineral replacement reactions, 2) formation conditions, and 3) geomechanical characteristics of progressively carbon dioxide-altered Coast Range rocks cropping out between Cholame and San Juan Bautista. Finally, three-dimensional time-dependent seismicity data along the SAF north of Parkfield will be analyzed to examine its correlation with mineralogical zoning.California has endured fatal, high-impact earthquakes, which claimed hundreds of lives and caused billions of dollars of damage; hence there is a critical need to understand how fluid-rock interactions influence the mechanical properties of the San Andreas Fault system. This project explores a new idea that might explain why certain portions of the fault exhibit aseismic creep whereas other portions are characterized by earthquakes. Ultimately, this study of carbonation of serpentinite will promote a deeper understanding of the seismicity and earthquake deformation cycles in one of the most densely populated regions of the United States.